This corresponds to what the GC3000 blob does. The USED / UNUSED enums are
wrong, at least for GC2000/GC3000.

Without this the 3rd texture component is not interpolated correctly (flat?)
in the following test (and others):

dEQP-GLES2.functional.texture.mipmap.cube.generate.rgba8888_nicest

Strangely, when the texture is sampled from OpenGL it works correctly,
the problem only shows up for sampling by gallium/blitter. This fixes other
cube map tests which use util_blitter_blit.

Signed-off-by: Jonathan Marek <jonathan@marek.ca>
Reviewed-by: Christian Gmeiner <christian.gmeiner@gmail.com>